A reliable method of sterilization includes utilizing saturated steam in a suitable pressure autoclave. High temperature dry heat has also been proven as a reliable method of sterilization. However, both of these methods of achieving sterilization are severely degrading to a wide variety of materials used in the medical and allied fields. For example, most plastics, virtually all paper based items, optical instruments, electronic items, adhesive products, and many essential medical/hospital products are all irreparably damaged by exposure to sterilizing conditions in saturated steam or dry heat. Although it is rarely used in hospitals, industrial use of various forms of radiation sterilization is common in many European countries, but the radiation doses commonly used have severe detrimental effects on many materials, particularly plastics such as vinyl (polyvinyl chloride) and Teflon (polytetra fluoroethylene).
In contrast to sterilization by utilizing heat and radiation, the use of conventional gaseous sterilization, particularly ethylene oxide (1, 2 epoxy-ethane) gas, has greatly reduced the adverse effects on even the most sensitive materials when it is used in the standard vacuum/pressure sterilization cycle in a pressure vessel. Even though these cycles are normally processed at temperatures slightly elevated above room temperature, the mild heat usually does not degrade most materials which are sensitive to heat. It is common for these moderate temperature vacuum/pressure cycles to inject additional water vapor to control the relative humidity within the pressure vessel and maintain it at a value as near saturation as possible. This is necessary in order to prevent or compensate for possible dehydration of bacterial spores which makes them extra-resistant to sterilization by ethylene oxide gas.
All of the above and other problems and compensations are largely avoided by sterilizing with ethylene oxide at room temperature. Room temperature ethylene oxide sterilization was heretofore made possible in economically practical and physically compatible form for use in hospitals by the introduction of a system of sterilization known as ANPROLENE (Registered Trademark of H. W. Andersen Products, Inc.,) as fully described in U.S. Pat. Nos. 3,476,506; 3,552,083 and in other pending patent applications. This known system of sterilization employs the methods of controlling a gaseous sterilant through the use of membranes of controlled permeability through which the ethylene oxide gas diffuses at a controlled and predictable rate. Wide experience in the hospital and associated medical fields has proven the suitability of room temperature ethylene oxide sterilization.
The use of gaseous sterilants in industrial processes is also widely known. Most of these utilize the familiar vacuum/pressure cycle in a pressure chamber designed for the purpose. The slightly elevated temperature generally used, and the rehumidification water vapor injected into the chamber, do not severely damage a wide variety of medical products. However, vacuum/pressure vessels suitable for industrial use are necessarily massive in construction in order to withstand the forces involved and are both costly to install and costly to operate. The physical characteristics of these known vacuum/pressure cycles requires packaging material which is capable of not only gently releasing the air entrapped within the package during the initial vacuum portion of the sterilization cycle, but of greater importance, must permit adequate penetration of the sterilant to every remote fold and crevice of the contents of the package. In order to absolutely assure that every microbe within the contents of the gas sterilization chamber has been reached and sterilized by the gas, cycle times must of necessity be long, typically 8 to as long as 24 hours. It is understandably costly to tie up these massive chambers for such long sterilization cycles. Where the product or unit being sterilized is small, the unit cost per cycle is acceptably low and economical, however, as unit size increases, the number of units which can be processed per sterilization cycle is reduced and the apportioned unit cost of sterilization and bacteriological sterilization controls increases rapidly.
An economical alternative to chamber sterilization is a process known as the STERIJET process (Registered Trademark of Sterilcoa, Inc.) which is described in U.S. Pat. Nos. 3,516,223; 3,630,665; 3,564,861 and 3,597,934. The STERIJET process produces vacuum packaged sterile items commonly used in the medical and allied fields, including some aspects of the food industry, for example, spices. Packaging material for the STERIJET process is usually flexible, conformal, and designed to enhance the skin tight appearance of a hermetically, vacuum sealed package. Even the slightest packaging defect or damage to the microbiological contamination barrier will be evidenced by the loss of the skin tight appearance of the vacuum packaged conformal membrane. This loss of vacuum tight appearance is a signal to the user or interim inspector that the sterility of the contents at that time must be suspect.
One of the design problems that had to be solved for this known STERIJET system is the sterilant gas control method used to assure that each unit that is vacuum packaged sterile receives exactly the correct predetermined quantity of gas sterilant. Among the possibilities, such as volumetric measurement and constant volume-variable pressure, the method of controlling delivered volume by use of constant pressure, constant system impedance to flow (an orifice) and variable time of sterilant gas flow has been found to be the most reproducible method of regularly delivering the same quantity of gas. This system requires that the system delivery pressure remain constant during the use of the machine. Several methods of achieving this are fully described in the aforementioned U.S. patents.
Ethylene oxide is capable of existing both as a liquid and a gas when confined under pressure. If the atmosphere within a pressure vessel is entirely ethylene oxide, it can exist both as a liquid and a gas, with an interface between the two. The pressure within the vessel will be entirely determined by the temperature of the liquid and the gas, assuming essentially isothermal conditions, and providing the contents of the vessel are entirely pure ethylene oxide. This physical phenomena is a well known characteristic of liquid/gaseous vapor interfaces, and need not be further explained in detail. Thus, it may be seen, if it is desired to control the pressure of such a system as a constant, it is necessary to control the temperature of the liquid gas interface and keep it constant.
However, if gaseous sterilant is withdrawn or delivered by the system, for example to packages to be sterilized, the instantaneous pressure during the withdrawal will decrease, causing some of the liquid to evaporate to gas to maintain the pressure of the gas. This evaporation causes a cooling of the liquid/gas interface, which reduces the temperature at the interface and hence the gas pressure above the interface. Under normal conditions, the quantity of heat stored in the balance of the liquid, restores the liquid gas interface to nearly its former temperature by thermal convection currents within the liquid. Repeated withdrawal of gaseous sterilant will cause a gradual but continued reduction of the temperature, and therefore the pressure within the vessel unless a quantity of heat is transferred to the liquid by some integral or external means that is sufficient to replace the heat of evaporation.
This heat of evaporation could be replaced by several means. For example, heaters could be placed directly in contact with the ethylene oxide; heat transfer fluids could be circulated either by thermal convection or by forced, pumped circulation, through heat exchange pipes in the sterilant tanks; the sterilant could be directed through an external heat exchanger as an evaporator or boiler and recondensing the vapor or gas in the tank, thus releasing the heat of condensation within the tank; the sterilant tank itself could be immersed in a heat transfer fluid, either as a closed or open system, depending on the heat transfer fluid chosen; heat could be transferred by radiant energy, either from a light or electromagnetic source; the sterilant tank could be heated by the condensation of a heat transfer fluid on the exterior of the sterilant tank or by condensing the heat transfer vapor within heat exchanger means within the tank; heat could be transferred by conduction, convection and radiation from electrical heaters placed on the outside of the sterilant holding vessel; and with varying degrees of success, by almost any heating method.
Most sterilants, such as ethylene oxide, propylene oxide and similar agents, are very reactive molecules. Of principle concern in the management, storage, and control of sterilants, are oxidation, self reaction such as polymerization, and possible reaction with the materials of construction used in the manufacture of storage, piping, and other sterilant system components.
Many sterilants, ethylene oxide included, are readily oxidizable, and thus flammable. Ethylene oxide, for example, is flammable in concentrations of 3% in air to 80% in air under normal conditions. It is of prime concern, therefore, that any sterilant system designed for the management of ethylene oxide be well designed to prevent ignition of ethylene oxide under both normal operating conditions, and in the event of any foreseeable equipment failure or malfunction. Prevention of ignition can be accomplished in several ways: removal of all sources of ignition, which is the usual precaution; encapsulation of sources of ignition in sealed housings which are impervious to flame fronts, including internal pressurization to prevent entry of flammables; enforced dilution which absolutely prevents attaining flammable concentrations; absolute containments of any possible leakage, either by use of mechanically secure systems which are usually all welded, or by containment of the complete system within an overenclosure which will contain and vent any leaks to a non-hazardous discharge point.
The present invention relates to a method and apparatus for controlling sterilant gas pressure. An additional feature of the present invention is that all components of the system that are under pressure, and hence might be a source of accidental leakage of the sterilant into the surrounding atmosphere, are contained within an absorbent medium capable of containing even large leakage rates, and signalling the fact of leakage by automatic audible or other alarm, giving more than adequate time for corrective action to be taken by supervisory or maintenance personnel.
According to the present invention a method of safely managing flammable gases, particularly sterilants such as ethylene oxide, involves completely immersing the pressurized portion of the system under an absorbtive fluid. In the case of ethylene oxide, an example of a suitable fluid is water, treated to enhance the absorption of ethylene oxide. Under standard conditions, water will absorb up to 4.5% of ethylene oxide by volume and the mixture will remain nonflammable. A specific feature of the method according to the present invention is the placing of an inverted open ended vessel over the components containing ethylene oxide under pressure such that the vessel is itself immersed in the fluid. The depth of immersion is not of substantial importance, however, the benefits to be obtained in this unique method are realized if initially, the immersion fluid is drawn completely into the inverted vessel by removing all air or other gases that may have been entrapped during the inverting. This may be accomplished by simple temporary venting or by withdrawing the gas by pressure differentials such as vacuuming.
It has been demonstrated that a leak of a sterilant such as ethylene oxide, which occurs under such an inverted immersed vessel will be quickly absorbed into the immersion fluid. If the leak occurs at a substantial rate, the gas continues to be absorbed into the surrounding fluid until the local dissolved concentration exceeds the limit of solubility, at which point the gas collects under the inverted open end vessel, forcing the locally saturated immersion fluid down in the inverted vessel. At the extreme, the entrapped gas will again pass under the lip of the inverted vessel where it will be absorbed in the surrounding fluid, which has not been locally saturated.
At each stage of this development there are physicochemical changes occurring which can be sensed to signal the existance of a leak. For example, the dissolving ethylene oxide will cause a change in the specific gravity of the immersion fluid which can be detected automatically. The pH of the fluid will also change. At the point where the local limit of solubility has been reached, the leaking sterilant will form a collection of gas. This may be detected by a change in buoyancy of the inverted vessel; a change in liquid level outside the immersed vessel; a change in the thermoconductivity of the fluid from a liquid to a gas. There are undoubtedly other changes of lesser importance.
According to the present invention, the immersion fluid is also the heat transfer medium. It may be maintained at a temperature which will ensure adequate sterilant pressure. Because the temperature of the sterilant supply will be reduced by the heat of evaporation as the sterilant is converted from its liquid storage state to its gaseous delivery state, the pressure of the sterilant will tend to decrease as its temperature decreases. This must be offset by heat transfer from the immersion fluid. This heat transfer can be accelerated by maintaining fluid circulation within the immersion medium. Thermal convection currents will exist, and it may be advantageous to augment these with forced flow by a pumping means.
In order to assure the most constant delivery pressure possible, the apparatus of this method also employs a self-compensating pressure regulator. It has been shown that it is necessary to maintain this pressure regulator within the temperature of the immersion fluid and heat transfer medium. If the pressure regulator and its associated dispensing valves are permitted to cool to a temperature lower than the condensation temperature, liquid ethylene oxide will collect in the piping at these cooler points and will be carried with the sterilant gas stream, thereby substantially increasing the amount of sterilant delivered to the package to be sterilized and possibly causing damage to the product from liquid sterilant's solvent action.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described in relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.